The field of the invention is in the high frequency radio receiver art and more particularly that of radar receivers having frequency measuring capability.
The conventional Instantaneous Frequency Measurement (IFM) receiver is a radio frequency (RF) receiver used primarily in electronic warefare (EW). Its basic function is to measure the frequency of pulsed signals radiated from hostile radar. Although some IFM receivers have the capability of measuring pulse amplitude, pulse width, and time of arrival, this invention is only concerned with the frequency measurement capability of the receiver.
Generally, it may be said that IFM receivers measure the frequencies of incoming RF signals utilizing interferometric techniques by detecting the phase shift magnitudes produced in multiple, calibrated delay lines. For instance, the received RF signal is divided and simultaneously introduced into a non-delayed path and a delay line of known length .tau.. Since the phase differences between the delayed and non-delayed receiver paths are functions of the input signal frequency, conversion of the phase difference signals to video provides signals whose amplitudes are related to the phase delay. These video signals typically take the form sin .omega..tau. or cos .omega..tau., where .omega. is the angular frequency of the processed input signal. The sin .omega..tau./cos .omega..tau. signals are delivered to the encoding network which makes amplitude comparisons of the signals, determines the numerical value of .omega., and generates the digital frequency descriptive word.
Characteristically, to achieve wide, unambiguous bandwidths and fine frequency resolution, it is necessary for the IFM receiver to have multiple delay lines, correlators and comparators to accomplish the frequency measurement. The delay between the leading edge of the RF pulse and the strobe to encode the sin .omega..tau./cos .omega..tau. video signals can be no shorter than the length of time it takes for the signal to transition the longest delay line, correlator and encoding network. As a general rule, the strobe to encode a sample occurs less than 120 nsec after the leading edge of the first RF pulse.
An IFM receiver has many attractive features necessary for EW applications, such as small size, light weight, wide instantaneous bandwidth, and fine frequency resolution. Unfortunately, a conventional IFM receiver has inherent signal detection problems when presented with time coincident received pulse signals. It is fairly common for many modern radars to simultaneously emit pulse signals of two or more frequencies, resulting in their arriving simultaneously at an intercept receiver. If the time difference between the leading edges of the two RF incoming pulses is greater than the time needed to complete the strobe encoding process (i.e., 120 nsec), the receiver will detect and frequency encode the leading RF signal without any problem. However, if the two RF pulses overlap with a time between leading edges less than the time to complete an encode strobe, one of three conditions will result. Either the first signal will be correctly encoded, or the second signal will be correctly encoded, or the receiver will encode ambiguous data (erroneous frequency data). Consequently, an IFM receiver can convey ambiguous frequency data when near-simultaneous RF pulses are received.
The probability of obtaining erroneous frequency data from an IFM receiver is also influenced by the relative power difference between the incoming RF pulses. The effects of amplitude differences are best understood with reference to the plots presented in FIGS. 1 and 2 representing actual results from a tested IFM receiver. FIG. 1 indicates the probability of encoding erroneous data (which is defined as more than 10 MHz away from either input signal) with respect to the power difference in two RF signals when there is zero time delay between their leading edges, namely, simultaneous RF pulses. In FIG. 1, the erroneous frequency data produced in the worst case is about 23%. FIG. 2 shows the effects of a 40 nsec lag between the leading edges of the first and second pulses. In FIG. 2, the worst case is about 25%.
Recognizing that even a small percent of erroneous frequency data can cause the signal processor following the IFM receiver to measurably slow or completely malfunction, it is essential that the existence of simultaneous or near-simultaneous RF pulses of different frequencies be detected. Once the concurrence is detected, the encoded frequency measurement can be disregarded or flagged for special handling.
The best known prior art is that contained by U.S. Pat. No. 3,939,411 to patentee James which discloses an IFM system which includes a technique to actually measure pulse signals emitted simultaneously. This technique utilizes a dispersive delay line which is not utilized in the present invention. A potential disadvantage exists with this technique when a series of input pulses is received which though separated at the input of the dispersive delay line, actually becomes simultaneous at the output. The James patent also does not flag the simultaneous pulse data as having a probability of erroneous data.
A patent application Ser. No. 176,434 now U.S. Pat. No. 4,336,541, was filed on Aug. 8, 1980, by James Tsui et al on a Simultaneous Signal Detection Circuit for an Instantaneous Frequency Measurement Receiver. The circuit in that application will detect simultaneous signals when the leading edges of the two signals are separated more than 20 nsec. When the leading edges of the two signals are time coincident or less than 20 nsec, the detection circuit does not sense the simultaneous signals. Clearly, a need exists to detect simultaneous signals with leading edge separation less than 20 nsec.